This invention relates to a pulse Doppler measuring apparatus and in particular to an apparatus for measuring the velocity of an object by using ultrasonic wave, e.g. a pulse Doppler measuring apparatus capable of measuring it with a high signal to noise ratio, in the case where the blood flow speed in a living body is measured in real time.
Heretofore various sorts of apparatuses are known for measuring the flow speed of an object by using the Doppler effect of acoustic wave. In particular, in an apparatus using the pulse Doppler method (cf. e.g. D. W. Baker; Pulsed Ultrasonic Doppler Blood Flow Sensing; IEEE Trans: Sonics and Ultrasonics; vol. SU-17, No. 3 July 1970, pp. 170-185), it is known that it is possible to identify a measured part by transmitting a pulsed continuous wave and setting a time gate corresponding to the distance to the measured part on the received signal.
As prior art ultrasonic Doppler blood flow measuring apparatuses, as disclosed in e.g. JP-A-No. 58-188433, JP-A-No. 60-119929 and JP-A-No. 61-25527, there are known apparatuses for measuring blood flow by transmitting ultrasonic wave towards blood vessel and measuring the Doppler shift frequency of the ultrasonic wave reflected by the blood in the blood vessel to obtain vcos.theta., where .theta. represents the angle between the direction of the blood flow and the transmission direction of the ultrasonic wave and v indicates the blood flow speed.
Further techniques, by which distribution of the blood flow speed in a certain cross-section in a living body is measured and displayed in color on a tomographic image, called color flow mapping, are disclosed in C. KASAI et al; Real-Time Two-Dimmensional Blood Flow Imaging Using an Autocorrelation Technique; IEEE Trans. Sonics and Ultrasonics, vol. SU32, No. 3, May 1985, pp. 458-464. For effecting this color flow mapping, in order to achieve a desired image frame rate, the blood flow speed at each of pixels is obtained by averaging measured values of the Doppler shift of a relatively small number of measurements. In the example described above, the auto-correlation method is used, by which a difference vector is obtained, each repeated measurement, by means of a auto-correlator between a vector indicated by a Doppler signal detected currently and a vector indicated by a Doppler signal detected the last time and the average speed is calculated by using the argument of a vector representing the sum of a plurality of difference vectors.
On the other hand, in U.S. patent application Ser. No. 101,444, filed Sept. 28, 1987, copending with this application, is disclosed a method, called 2-axial-component method, by which measurements being repeated, a phase difference .DELTA..theta. of the Doppler signal is obtained for every measurement, which difference is decomposed into a cosine component and a sine component; a plurality of values obtained for each of the components are added and averaged; and the phase difference indicated by the averaged cosine and sine components thus obtained is transformed into the velocity.
Furthermore, in 1978 Ultrasonic Symposium Proceedings, pp. 348-352 is disclosed a method, by which a phase difference of the Doppler signal is obtained for every measurement and an averaged phase difference is calculated by adding directly a plurality of values of the phase difference, which averaged phase difference is transformed into the velocity. Hereinbelow this is called phase difference averaging method.